CAMBRIDGE, Mass., Feb. 6 /PRNewswire-FirstCall/ -- Idenix
Pharmaceuticals, Inc. (Nasdaq: IDIX), a biopharmaceutical company engaged
in the discovery and development of drugs for the treatment of human viral
and other infectious diseases, today reported data for IDX899, a
non-nucleoside reverse transcriptase inhibitor (NNRTI) being developed for
the treatment of HIV. In the first dosing cohort of an ongoing phase I/II
study, eight HIV-1 infected treatment-naive patients receiving 800 mg of
IDX899 once-daily achieved a mean reduction in virus level of 2.01 log(10),
or 99 percent, after seven days of treatment. Additionally, two posters
detailing the in-vitro resistance and pharmacokinetic profile in man of
IDX899 were presented at the 2008 Conference on Retroviruses and
Opportunistic Infections (CROI) this week in Boston, MA.

"New once-a-day NNRTIs that offer improved resistance and safety
profiles over what is currently available would be a valuable asset to
HIV-treating physicians," said Dr. Douglas Richman, Professor of Pathology
and Medicine, University of California San Diego, and Director of the UCSD
Center for AIDS Research. "The early profile of IDX899 shows promise and
warrants continued clinical evaluation as a potential HIV therapy."

Interim Proof of Concept Data in HIV-infected Patients

An ongoing phase I/II clinical trial is evaluating the safety,
tolerability and antiviral activity of IDX899. In the first cohort of the
study, ten HIV-1-infected treatment-naive patients were randomized 8:2 to
receive once-daily 800 mg IDX899 or matching placebo, respectively, for
seven days.

Patients receiving once-daily 800 mg of IDX899 achieved a mean and
median plasma viral load reduction of 2.01 and 2.11 log(10), respectively,
after seven days of treatment. Six out of eight patients achieved a 2
log(10) or greater reduction in viral load with one patient achieving
undetectable virus levels (< 50 copies/mL). No serious adverse events were
reported in this cohort and no patients discontinued the study. Given the
potent antiviral activity and favorable preliminary safety demonstrated at
800 mg once-daily, we will explore sequential cohorts of 400 mg once-daily
followed by 200 mg once-daily.

"We are pleased with the safety profile and potency observed with the
800 mg dose of IDX899 in HIV-infected patients and based on these data we
will continue to evaluate lower dosing regimens in order to optimize the
role of IDX899 in HIV combination therapy," said Douglas Mayers, M.D.,
Idenix's chief medical officer.

In-vitro Resistance Data

In a preclinical resistance study, IDX899 and other marketed and
investigational NNRTIs were compared to evaluate in-vitro genotypic
resistance and phenotypic cross-resistance profiles. IDX899 demonstrated
potent antiviral activity against established NNRTI-resistant clinical
isolates. Compared to efavirenz (Sustiva(R)), the emergence of
IDX899-resistant HIV-1 isolates was slower and required several mutations
suggesting a higher barrier to resistance for IDX899. The resistance
mutations selected in-vitro with IDX899 were different from those selected
with efavirenz. Efavirenz appeared to be active against IDX899 resistant
viruses and IDX899 remained active against efavirenz-resistant virus
containing as many as four NNRTI-resistance mutations.

A phase I study was conducted to evaluate the safety and
pharmacokinetics of IDX899 following single escalating (n=65) and multiple
(n=20) doses in healthy volunteers. In this study, following once-daily
oral administration, IDX899 appeared to be well tolerated at single doses
up to 1200 mg and multiple doses up to 800 mg over a seven-day period. No
serious adverse events or pattern of laboratory abnormalities were
observed. Food enhanced the absorption of IDX899. Additionally, in a
single-dose drug-drug interaction study assessing the combination of either
100 or 800 mg IDX899 and 100 mg atazanavir (Reyataz(R)), atazanavir plasma
levels were not markedly altered by IDX899.

About Idenix

Idenix Pharmaceuticals, Inc., headquartered in Cambridge,
Massachusetts, is a biopharmaceutical company engaged in the discovery and
development of drugs for the treatment of human viral and other infectious
diseases. Idenix's current focus is on the treatment of infections caused
by hepatitis C virus and HIV. For further information about Idenix, please
refer to http://www.idenix.com.

Forward-looking Statements

This press release contains "forward-looking statements" within the
meaning of The Private Securities Litigation Reform Act of 1995. Such
forward- looking statements can be identified by the use of forward-looking
terminology such as "could," "may," "will," or similar expressions, or by
express or implied statements with respect to the company's clinical
development programs in HIV, or any potential pipeline candidates for the
treatment of HIV, including any expressed or implied statement regarding
the efficacy and safety of IDX899. Such forward-looking statements involve
known and unknown risks, uncertainties and other factors that may cause
actual results to be materially different from any future results,
performance or achievements expressed or implied by such statements. There
can be no guarantees that the company will advance any clinical product
candidate or other component of its potential pipeline to the clinic, to
the regulatory process or to commercialization. In particular, management's
expectations could be affected by unexpected regulatory actions or delays;
uncertainties relating to, or unsuccessful results of, pre-clinical studies
and/or clinical trials, including additional data relating to the ongoing
pre-clinical studies and/or clinical trials evaluating its product
candidates; the company's ability to obtain additional funding required to
conduct its research, development and commercialization activities; the
company's dependence on its collaboration with Novartis Pharma AG; changes
in the company's business plan or objectives; the ability of the company to
attract and retain qualified personnel; competition in general; and the
company's ability to obtain, maintain and enforce patent and other
intellectual property protection for its product candidates and its
discoveries. These and other risks which may impact management's
expectations are described in greater detail under the caption "Risk
Factors" in the company's annual report on Form 10-K for the year ended
December 31, 2006 and the Quarterly Report on Form 10-Q for the quarter
ended September 30, 2007, each as filed with the Securities and Exchange
Commission (SEC) and other filings that the company makes with the SEC.

All forward-looking statements reflect the company's expectations only
as of the date of this release and should not be relied upon as reflecting
the company's views, expectations or beliefs at any date subsequent to the
date of this release. Idenix anticipates that subsequent events and
developments may cause these views, expectations and beliefs to change.
However, while Idenix may elect to update these forward-looking statements
at some point in the future, it specifically disclaims any obligation to do
so.

Sustiva(R) and Reyataz(R) are registered trademarks of Bristol-Myers
Squibb Company.

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